Visual hydrometers which indicate the liquid level and the specific gravity of a liquid are well known for many applications, including checking of storage batteries. The simplest manner in which this is achieved is by use of an elongated light transmitting view rod which has a conical lower end that extends into a channel which contains a float. The float is generally a ball of a bright color so that when it is adjacent the conical end of the viewing rod the observer looking down at the top surface of the rod will see the colored float. Thus, when the observer sees the float it indicates to him that the liquid level is adequate and that the specific gravity of the liquid is appropriate. If the float falls in the channels sufficiently far below the conical end of the rod so that it may no longer be viewed, it indicates to the observer that the liquid level is low when a bright indication is presented or that the specific gravity of the liquid is low when a dark indication is presented.
Electronically monitored hydrometers are also known in which a light beam between a light source and a light detector are interrupted by a float. A conventional manner of implementing such a system is to mount the light source and the light sensor in a part of a cap which extends above the liquid, and to transmit the light down one light pipe to a reflector which reflects the light across a channel in the liquid that contains the float to a second reflector which directs the light up a second light pipe to a light detector. The position of the float determines whether the light beam is interrupted so that the liquid level or the specific gravity of the liquid may be electronically monitored at a remote location.
The difficulty of sealing the sensor and the light source in an electronic hydrometer from the liquid being sensed has encouraged the aforementioned construction in which the light source and detector are positioned at a location which is removed from the liquid. This construction results in a bulkier configuration for the hydrometer and requires additional parts such as the light pipes and the reflecting prisms, thereby noticeably increasing the cost of the unit. Moreover, while prior electronic liquid monitoring indicators were capable of lighting a warning light, such as a light on the dash board of an automobile, to indicate that its battery was not operating properly, these indicators did not provide for a way of checking whether or not the electronic monitoring system was itself operating properly. Moreover, prior electronic liquid monitoring indicators required electrical current from the battery in order to operate. When a faulty battery was present in a vehicle which would not start at all, the electronic monitoring system of the vehicle became useless unless it was supplied from an auxillary power source. With the advent of sealed batteries, the task of locating the source of a battery failure became even more difficult. However, with the combined visual and electronic indicator of the present invention, even a sealed battery may be checked visually. Other advantages of the hydrometer of the present invention are that the light detector may be positioned so that it is directly in line with the light source, thereby allowing a less intense light source and lower power dissipation, and that the light source and light detector are sealed from the liquid being monitored by an outer housing which supports the visual viewing rod.